A liquid crystal display (denoted LCD) drives a plurality of active devices, such as thin film transistors, by a plurality of scan lines and writes data into the pixel electrodes by the data lines. The different lengths of the wires dissipate and delay the voltage waveforms so that a wrong data is written into the pixel electrodes. Additionally, a parasitical capacitor caused by the material or manufacturing also distorts the voltage waveforms by a feedthrough voltage generated by the parasitical capacitor. It is impossible to fix and unify the feedthrough voltage during manufacturing, so how to conquer the feedthrough voltage to diminish the flicker of a LCD is important.
FIG. 1 is a diagram showing the equivalent circuit of part of a known TFT-LCD. Gate driver 200 drives the scan lines G1 . . . Gn−1 sequentially to switch on the thin film transistors (denoted TFTs), and the source driver 100 writes a data (voltage) to the data lines S1 . . . Sn−1. For example, a scan line G1 and a data line S1 electrically connect to a TFT 400 of a pixel, in which the gate electrode, the source electrode and the drain electrode of the TFT 400 connect to the scan line G1, data line S1 and pixel electrode respectively. The pixel includes a storage capacitor (Cst) 410 and a liquid crystal capacitor (Clc) 420, and the storage capacitor (Cst) 410 maintains the voltage on the pixel electrode until that the scan driver drives the scan line again, and the liquid crystal capacitor (Clc) 420 provides a voltage across the liquid crystal in the pixel. The pixel electrode couples a common electrode to form the liquid crystal capacitor (Clc) 420 to provide the voltage across the liquid crystal, which is pixel voltage.
FIG. 2 is a diagram showing the equivalent circuit of a pixel. The gate electrode connects to a scan line 210, the source electrode connects to a data line 110, and the drain electrode connects to a pixel electrode to write the data into the pixel electrode. As shown in the diagram, there exists a parasitical capacitor between the gate electrode and the drain electrode of the TFT 400.
FIG. 3 is a diagram of voltage waveforms illustrating the voltage variation of a pixel electrode. When the scan line voltage 440 raises from Vgl to Vgh to switch on a TFT 400, the data line voltage 450 charges the pixel electrode during a duty time Ton, so the pixel voltage 460 rises up from Vdl to Vdh. After the duty time Ton, the scan line voltage 440 goes down to Vgl to switch off the TFT, and the data line voltage 450 falls from Vdh to Vdl. Since the storage capacitor holds the pixel voltage 460, so the pixel voltage 460 does not fall to Vdl. Theoretically, the pixel voltage 460 should hold at Vdh, but a parasitical capacitor Cgd pulls down the pixel voltage 460 for a feedthrough voltage ΔVp. The voltage difference between the pixel voltage and the common voltage 470 on the common lines shifts for a feedthrough voltage ΔVp to flicker the screen of a TFT-LCD.
For diminishing or vanishing the flicker, US. App. No. 2005/0018121 discloses a teaching of zigzagging the wires between data lines and source driver or between scan lines and gate driver to a similar length to cancel out the wire delay, but does not eliminate the parasitical capacitor.
Next, U.S. Pat. No. 6,933,917 discloses a teaching of connecting the scan lines to control circuits to provide impedance. Each control circuit connects a scan line to a transistor, where the gate electrode of the transistor connects a variable resistor and then to a power supply, one electrode to a common line. The impedance generated by the control circuit is much larger than that generated by the TFT of a pixel, so that, in relatively, the impedance generated by the TFT of a pixel can be neglected almost. It means the feedthrough voltage ΔVp decreases relatively to diminish the flicker.
The feedthrough voltage ΔVp varies from pixel to pixel, so the same impedance cannot diminish all flickers on the screen of a TFT-LCD. It is still an important topic to develop a new skill to solve this problem.